At the beginning of this year I was using, abusing and tough testing the Osprey Transporter pack, which is essentially a 30L ...
The lightest softside medium checked suitcases weigh between 4.5-6 lbs (2-2.7 kg) and they're made from polyester or nylon.
No backpack is ever fully waterproof unless it’s a dry bag design, but Osprey’s Antigravity is definitely water-resistant – on test I found that rain beaded right off the surface ...